A carrier network for providing services of connection to the Internet consists of an access network for holding a plurality of subscribers directly and a relay network (hereunder, to be referred to as a metro network) for bundling a plurality of access networks. And a passive optical network (PON) is one of such access network types.
A PON is a network system for connecting an OLT and a plurality of ONUs to each other through an optical fiber network provided with a function for branching an optical signal and enabling each ONU to transmit data to the OLT in a time division manner according to its access right (data transmission time band) information notified from the OLT. The PON branches an optical fiber connected to an OLT into a plurality of branch line optical fibers with use of an optical splitter (optical coupler) to enable an optical fiber section between an optical splitter and the OLT to be shared by a plurality of ONUs when an ONU disposed in the user's house is connected to a branch line optical fiber. Thus the PON can reduce the expenses for laying the optical fiber and hold many user terminals through the ONU.
The OLT of the PON system is connected to a metro network through, for example, a gateway (GW). The GW is a frame transfer device for holding a plurality of OLTs and transferring each variable length frame between each OLT and the metro network according to its header. In addition to the GW for housing the OLT, the metro network also includes another GW for housing, for example, an ISP network. Consequently, each user terminal connected to an ONU can access the Internet through the OLT, GW, and ISP network.
There are some types of PONs such as B-PON (Broadband PON) for transmitting information with use of fixed length ATM cells in an optical fiber section (PON section), G-PON (Gigabit PON) capable of realizing gigabit class high speed data transfer, and GE-PON (Giga-Ethernet PON) preferred to Ethernet (trade mark) services.
In the case of the G-PON capable of transferring variable length frames, an OLT generates GTC (G-PON Transmission Convergence) downstream frames specific to a PON section and maps GEM (G-PON Encapsulation Mode) frames including packet data addressed to each ONU in the payload of each GTC downstream frame. The GTC downstream frames are broadcast to a plurality of branch line optical fibers through an optical splitter (optical coupler). Each ONU checks the ONU identification information (ONU port ID) of the destination indicated by the header part of the GEM frame extracted from the GTC payload to receive the packet data addressed to itself selectively, then transfers the data to the subject target user terminal.
The PON OLT has a DBA (Dynamic Bandwidth Allocation) function for allocating a data transmission time band (time slot) for each ONU according to an amount of accumulated transmission data (transmission queue length) in each ONU. In the DBA, a time slot is allocated to one or a plurality of ONUs in a predetermined cycle ΔT. Consequently, the maximum number of time slots to be allocated to one ONU becomes ΔT. If B is assumed as a bandwidth of an optical fiber line, therefore, the maximum bandwidth B (limit) to be allocated to one ONU becomes ΔT×B. The OLT DBA function distributes the maximum bandwidth B (limit) to be allocated in a cycle ΔT to a plurality of ONUs according to the amount of the data in the transmission queue in each ONU.
In the case of the G-PON, each time slot is specified with a transmission starting time and a transmission ending time. The OLT sets the bandwidth control information of each ONU indicating both ONU identification information and a time slot in the header part of a GTC downstream frame, then notifies the information items to each ONU. Because the length varies among the branch line optical fiber sections in the PON, the transmission delay time of frames streaming from ONU to OLT also varies among ONUs. Consequently, the OLT measures the transmission delay time of each ONU and notifies an equivalent delay time to each ONU beforehand. Each ONU has a function for correcting the specified transmission starting time with the equivalent delay time upon receiving an allocated time slot from the OLT to start data transmission at a proper timing.
The OLT transfers upstream frames received from the ONU to the GW. In this case, if the OLT transmission buffer is insufficient in capacity, the frames might be discarded. To avoid such frame discarding, for example, JP-A No. 159203/2004 discloses a packet transfer device for limiting the transmission data amount from every ONU housed in the subject OLT by adjusting the access rights in a PON section in case where the accumulated data amount in the OLT built-in buffer exceeds a predetermined threshold value. In the description below, congestion is defined as a state in which frames may be discarded if an amount of accumulated data that exceeds a predetermined threshold value is left as is.
In the field of communication networks, a back pressure (BP) technique is known well as a technique for preventing such discarding of communication frames. According to the technique, congestion occurrence is notified from a congestion-occurred node device to a data transmission source device so that the data transmission source device stops data transmission or adjusts the transmission data amount. For example, JP-A No. 153505/2004 discloses a data frame transmission system that controls congestion with use of a pose frame conforming to the IEEE (Institute of Electrical and Electronic Engineers) 802. 3. In JP-A No. 153505/2004, if congestion occurs, the subject data frame transmission device transmits a pose frame to the data transmission source device. Receiving the pose frame, the source device stops the data frame transmission or limits the bandwidth during a period specified with the payload part of the pose frame.